Fuel cells have high power generation efficiency and do not exhaust harmful matter, the fuel cells have been practically used as an industrial power generation equipment and a household power generation equipment, or as a power source for an artificial earth satellite, a space ship or the like. Further, in recent years, the fuel cells have been progressively developed as a power source for a vehicle such as a passenger car, a bus, and a cargo truck. Such fuel cells are categorized into an alkali aqueous solution fuel cell, a phosphoric-acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, a direct methanol fuel cell and the like. Specially, a solid polyelectrolyte DMFC (Direct Methanol Fuel Cell) is able to provide a high energy density by using methanol as a fuel hydrogen source. Further, the DMFC does not need a reformer and thus is able to be downsized. Thus, the DMFC as a small mobile fuel cell has been progressively researched.
In the DMFC, an MEA (Membrane Electrode Assembly) as a unit cell in which a solid polyelectrolyte film is sandwiched between two electrodes, and the resultant is joined and integrated is used. One gas diffusion electrode is used as a fuel electrode (anode), and methanol as a fuel is supplied to the surface of such one gas diffusion electrode. As a result, the methanol is decomposed, hydrogen ions (protons) and electrons are generated, and the hydrogen ions pass through the solid polyelectrolyte film. Further, the other gas diffusion electrode is used as an oxygen electrode (cathode), and air as oxidant gas is supplied to the surface of the other gas diffusion electrode. As a result, oxygen in the air is bonded with the foregoing hydrogen ions and the foregoing electrons to generate water. Such electrochemical reaction results in generation of electro motive force from the DMFC.
Meanwhile, in a fuel cell used for mobile purposes, it is desired that the fuel cell stably perform power generation operation in any environment, such as indoors, outdoors in midwinter, inside an automobile at high midsummer temperatures, and inside a bag where heat release is difficult. Further, it is also desired that the fuel cell be able to follow sudden changes in the environment, such as the fuel cell suddenly being carried from inside a warm room to freezing outdoors. In this way, since suitable fuel supply amount for the fuel cell differs according to the temperature and humidity of the external environment, careful fuel supply control according to environment changes (fuel supply control in which the fuel supply amount is not excessive or insufficient) is desired.
In a case where the supply amount of fuel becomes excessive, the surplus fuel permeates to the oxygen electrode, thereby causing a phenomenon called crossover. The crossover phenomenon is a phenomenon in which the surplus fuel burns directly on the oxygen electrode, thus not only reducing usage efficiency of fuel and causing waste, but also carrying risk of causing burn injury to a user resulting from temperature rise. In addition, on the contrary, in a case where the fuel supply becomes insufficient, sufficient output is not able to be obtained, and there is a possibility that power supply to equipment connected to the fuel cell is stopped.
Thus, a method of controlling the fuel supply amount for the purpose of inhibiting excess and shortage in the fuel supply amount has been proposed (for example, Patent Document 1).
In fuel cell systems including a fuel cell such as the foregoing, there is a fuel cell system in which a power generation voltage and a power generation current (generated power) from the fuel cell charge a secondary battery and drive a load. Thereby, in such a fuel cell system, it is desired that the generated power from the fuel cell charges the secondary battery as efficiently as possible.
In Patent Document 2, a fuel cell system in which control is performed so that the power generation voltage value of the fuel cell is held constant using a DC/DC converter is proposed.